1. Field of the Invention
The present invention relates to a computer-aided imaging diagnostic processing apparatus and computer-aided imaging diagnostic processing method which can satisfactorily visualize a desired target by properly setting parameters in volume rendering (VR) processing.
2. Description of the Related Art
Currently, in Japan, lung cancer has become the first cause of malignant tumor death and has increased steadily. This leads to strong social demands for not only prevention by anti-smoking measures but also early detection. In Japan, each municipality has practiced lung cancer screening by plain chest radiograph and sputum cytology. A 1998 report by “Study Group Concerning Cancer Screening Effectiveness Evaluation” of the Ministry of Health and Welfare concluded that current lung cancer screening had little effect if any. X-ray computed tomography (CT) can detect a lung field type lung cancer more easily than a plain chest radiograph. However, this technique could not be used for screening because it took a long imaging time before the advent of a helical scan type CT in 1990. Shortly after the advent of helical CT, a method of imaging at a relatively low X-ray tube current (to be referred to as low-dose helical CT hereinafter) has been developed to reduce exposure to radiation, and pilot studies have been made on lung cancer screening using this technique in Japan and the U.S. The study results have demonstrated that low-dose helical CT has a higher lung cancer detection rate than plain chest radiograph.
An increase in the number of CT detector rows after 1998 has shortened the time required for helical CT imaging. The latest multi-detector helical CT, such as 64 rows model, can image the entire lungs with an almost isotropic resolution of less than 1 mm in less than 10 sec. Technical innovation of CT raises the possibility that it can detect smaller lung cancers. However, the multi-detector helical CT generates roughly thousand images per scan, and hence the load required for interpretation of radiograms greatly increases.
When a volume rendering image of a lung field is to be displayed by using a conventional computer-aided imaging diagnostic processing apparatus, the apparatus uses a constant lung field opacity curve (opacity characteristic curve) regardless of density differences in the lung field or individual density differences in the lung field. In order to obtain a desired volume rendering image, therefore, the user adjusts parameters (an opacity curve and the like) for each volume rendering operation by using an imaging display apparatus user interface.
In volume rendering display of a CT lung field, it is necessary to obtain feature amounts of a volume of interest (VOI) to be displayed and determine parameters for volume rendering display on the basis of these feature amounts. It is, however, difficult to obtain a volume rendering image desired by the user by using these feature amounts, and hence it has required a very long time to adjust parameters by using an imaging display apparatus user interface. This increases the load on a doctor who interprets radiograms. For this reason, this technique is rarely used for diagnosis. That is, volume rendering image generation corresponding to each purpose is rarely used for diagnosis because it further increases the load on the doctor who interprets radiograms in spite of the fact that it can generate isotropic three-dimensional image data.
Under the circumstances, in order to establish low-dose helical CT as a lung cancer screening method and allow the use of volume rendering images for imaging diagnosis, demands have arisen for a method of easily displaying volume rendering images corresponding to purposes.